With the recent spread of personal computers, digital still cameras and digital video cameras are spreading remarkably. Images sensed by an image sensor have a noise pattern produced by the influence of a dark current flowing through the sensor. This noise is called dark current noise. The darker the screen, the more conspicuous the dark current noise. Generally, the influence of this dark current noise is said to be larger in MOS image sensors than in CCD image sensors presently widely used. A conventional image sensing apparatus for removing the dark current noise will be described below with reference to FIG. 8.
FIG. 8 is a schematic view showing the arrangement of a conventional digital still camera. FIG. 9 is a schematic view showing the configuration of a memory 52 of the camera. Reference numeral 41 denotes an optical lens for forming an optical image of an object; 42, a stop; 43, a shutter; 44, a driving circuit for individual mechanical parts; 45, a CCD as an image sensor for converting an optical image of an object into an electrical signal; 46, a timing signal generator (to be referred to as a TG hereinafter) for generating a timing signal required to drive the CCD 45; 47, an image sensor driver for amplifying the signal from the TG 46 to a level necessary to drive the CCD 45; 48, a CDS (Correlation Double Sampling) circuit for removing the output noise from the CCD 45; 49, an AGC (Auto Gain Control) circuit for amplifying the output signal from the CDS circuit 48; and 50, a clamp circuit for clamping the zero level of the amplified signal. Reference numeral 51 denotes an A/D converter for converting an analog signal into a digital signal; 52, a memory for storing an image sensing signal; 53, a subtracter for subtracting dark current noise from the readout image signal from the memory 52; 54, a signal processor for processing the image signal from which the dark current is removed, thereby converting the signal into a signal suited to storage in a recording medium; and 55, a recording medium such as a memory card or hard disk. Reference numeral 56 denotes an interface circuit for recording a signal on the recording medium 55; 57, a signal processing control CPU for controlling the signal processor 54; 58, a CPU for controlling the mechanism and an operation unit; 59, an operation display for displaying information for assisting operations and displaying the camera status; 60, an operation unit for externally controlling the camera; and 61, an AE light receiving unit for measuring the luminance of an object to calculate an F-number and a shutter speed for correct exposure.
The operation of this conventional digital camera shown in FIG. 8 will be described below. When a user instructs the camera to start photographing by the operation unit 60, the camera starts photographing. First, the AE light receiving unit 61 measures the luminance of an object. On the basis of the measurement value, the CPU 58 for controlling the mechanism and the operation unit calculates an appropriate F-number and shutter speed. Next, an image signal is read by driving the CCD 45 with the shutter 43 being closed. The CDS circuit 48 removes noise from the read image signal, and the AGC 49 amplifies the signal as needed. The clamp circuit 50 clamps the zero level of the signal, and the A/D converter 51 converts the signal into a digital signal. This digital signal is stored in the memory 52. Since the shutter 43 is kept closed, the CCD 45 presumably senses no image. The image signal thus stored in the memory 52 is a noise pattern produced by the influence of a dark current, i.e., a dark current noise component.
Subsequently, the CPU 58 for controlling the mechanism and the operation unit controls the mechanism driving circuit 44 on the basis of the calculated control value, thereby controlling the amount of light passing through the stop 42 and the opening time of the shutter 43. That is, an optical image of the object is formed on the image area of the CCD 45 with an appropriate light amount corresponding to the opening time of the shutter. The CCD 45 is driven by a driving signal which the image sensor driver 47 forms by amplifying the output from the TG 46. The CDS circuit 48 removes noise from the read image signal, the AGC 49 amplifies the signal as needed, and the clamp circuit 50 clamps the zero level of the signal. The A/D converter 51 then converts the signal into a digital signal, and this digital signal is stored in the memory 52.
After that, the image signal and the dark current noise component stored in the memory 52 are read out. The subtracter 53 subtracts the dark current noise component from the image signal. The signal processor 54 processes this image signal from which the dark current noise component is canceled. The processed signal is recorded on the recording medium 55 via the interface circuit 56.
Generally, the dark current noise component of an image sensor such as a CCD or MOS depends upon the temperature, i.e., it is strongly influenced by the temperature of the sensor and its ambient temperature. Hence, this dark current noise component varies with time after the power supply of the camera is turned on. Accordingly, the dark current noise component read by driving the CCD 45 with the shutter 45 being closed and the image signal exposed and read by opening/closing the shutter 43 are desirably obtained with a short interval between them. In the above conventional image sensing apparatus, therefore, a dark current noise component is read from the CCD 45 and stored in the memory 52 immediately before photographing, and a photographed image is corrected by using this stored dark current noise component. Another image sensing apparatus reads a dark current noise component from the CCD 45 on a predetermined cycle and updates a dark current noise component stored in the memory 52.
In the method in which a user operates the operation unit 60 to instruct the camera to start photographing and a dark current noise component is read out from the CCD 45 in response to this instruction, a considerable time elapses from the time the photographing start instruction is entered to the time the photographing is actually started. Also, in the method in which a dark current noise component is read out from the CCD 45 and stored in the memory 52 on a predetermined cycle, if read and storage of a dark current noise component are being executed when the camera is instructed to start photographing, a considerable time elapses before the photographing is actually started. As described above, a user may lose a shutter chance when using the conventional image sensing apparatus.